1. Field of the Invention
This invention relates to solid electrolytic capacitors using conducting polymer layers as solid electrolytes, and a method of making the same.
2. Prior Art
With the progress of science and technology, a reduction in the size of electronic equipment and an improvement in the reliability thereof are being desired. In the field of capacitors, there is a growing demand for high-capacitance solid electrolytic capacitors having good characteristics even in a high frequency region and, moreover, a high degree of reliability. In order to meet this demand, research and development are being actively carried on.
Usually, a solid electrolytic capacitor has a structure in which a porous compact of an electrochemical valve metal such as tantalum or aluminum is used as a first electrode (anode), an oxide film thereof as a dielectric, and a solid electrolyte such as manganese dioxide (MnO.sub.2) or a 7,7',8,8'-tetracyanoquinodimethane (TCNQ) complex as part of a second electrode (cathode). In this case, the solid electrolyte must perform the function of electrically connecting the whole dielectric surfaces within the porous compact with an electrode lead, and the function of repairing electrical short circuits arising from defects of the dielectric film. Consequently, metals having a high electric conductivity but lacking the dielectric repairing function cannot be used as solid electrolytes. For this reason, manganese dioxide which is converted into an insulator, for example, upon exposure to heat generated by a short-circuit current has been used. However, in capacitors using manganese dioxide as part of the cathode, the impedance in a high frequency region is not lowered because of the insufficient electric conductivity of manganese dioxide. On the other hand, capacitors using a TCNQ complex as part of the cathode have poor heat resistance because TCNQ complex are liable to thermal decomposition.
Recently, the development of new materials in the field of polymers has made considerable progress. As a result, conducting polymers comprising conjugated polymers (e.g., polyacetylene, poly-p-phenylene, polypyrrole and polyaniline) doped with an electron-donating or electron-attracting compound (dopant) have been developed. Among others, five-membered heterocyclic compounds (e.g., polypyrrole and polythiophene) and polyaniline can easily yield conducting polymers by electrolytic polymerization, and such conducting polymers are being used as solid electrolytes for capacitors (Japanese Patent Laid-Open Nos. 36012/'89 and 64013/'91). However, this method involves electrolytic polymerization on an oxide film having electrical insulating properties, so that it has been very difficult to form a uniform conducting polymer film.
Accordingly, a method which comprises first forming an electrically-conductive precoat layer and then forming a conducting polymer on the oxide film by electrolytic polymerization is being extensively employed (Japanese Patent Laid-Open Nos. 32619/'89, 36012/'89, 74712/'89, 225110/'89, 117121/'90, 64013/'91, 304055/'93 and 45200/'94). However, this method has the disadvantage that an auxiliary electrode must be disposed in proximity to the capacitor device, resulting in very low mass productivity.
There has also been proposed a method for applying a conducting polymer soluble in an organic solvent and then drying it to form a polymer layer useful as a solid electrolyte. For example, it has been proposed to make a solid electrolytic capacitor using polyaniline as the solid electrolyte according to a method in which a solution of previously prepared polyaniline is applied onto the surface of an oxide film of a metal and then dried to form a layer of polyaniline (Japanese Patent Laid-Open No. 35516/'91). However, this method has the disadvantage that the polyaniline solution has very high viscosity and fails to permeate throughout an oxide film having a finely porous structure and hence a highly extended surface area. As a result, in the capacitors made by this method, the capacitance appearance factor (i.e., the ratio of the actual value of electrostatic capacity to the design value) is significantly low.
On the other hand, a method for forming polyaniline by polymerizing aniline monomer on an oxide film is known. In this case, a satisfactorily high capacitance appearance factor can be achieved. However, since polyaniline itself has a lower electric conductivity than polypyrrole, the capacitors made by this method have the disadvantage that their characteristics in a high frequency region are poorer than those of capacitors using polypyrrole.
Moreover, a method for forming polypyrrole by polymerizing pyrrole monomer on an oxide film is also known. However, since polypyrrole is not be easily formed in the central part of a porous body, it is difficult to coat an oxide film having a finely porous structure completely with polypyrrole. As a result, the capacitor thus obtained has the disadvantage of showing a low capacitance appearance factor.
Furthermore, a method for forming a derivative of polythiophene on an oxide film has been proposed (Japanese Patent Laid-Open No. 15611/'90). The polymer formed by this method undergoes a high degree of shrinkage and hence tends to peel from the capacitor device, resulting an increase in impedance. Moreover, since the oxide film is exposed in the parts where the polymer has peeled off, the device becomes mechanically weak against external stresses and tends to show an increase in leakage current. Thus, it is impossible to secure the reliability of the device.
Furthermore, it has been proposed to use two layers of conducting polymer having different properties as solid electrolytes and thereby make the most of the characteristics of each compound (Japanese Patent Laid-Open Nos. 45481/'95, 45199/'94 and 45201/'94). Specifically, according to this method, the capacitance appearance factor of a capacitor can be enhanced by forming relatively easily formable polyaniline in the inside of the device and then forming polypyrrole thereon, or the heat resistance of a capacitor can be improved by forming polypyrrole having a high electric conductivity in the inside of the device and then forming thereon polyaniline having high heat resistance. In the former case (polyaniline/polypyrrole), the capacitance appearance factor is enhanced, but the impedance in a high frequency region is not sufficiently lowered because of the low electric conductivity of polyacetylene. In the latter case (polypyrrole/polyaniline), the heat resistance is improved, but the capacitance appearance factor is reduced because polypyrrole cannot be easily formed in the inside of the device.
As described above, conventional solid electrolytic capacitors have involved difficulty in improving their capacitance appearance factor, frequency characteristics, heat resistance and reliability at the same time.